Field of the Invention
This invention relates to cam-lock fastening systems for attachment of components, and more particularly to a cam-lock fastening system; providing for a structural and electrical interface that facilitates rapid attachment, removal and reattachment of mechanical and electro/mechanical components with tight alignment tolerances. The cam-lock fastening system can be used, for example, to facilitate “plug and play” of different missile payloads to a common airframe.
Description of the Related Art
A cam-lock fastening system is used to fasten a pair of structures and to provide a clamping load or “preload” at the surface between the structures. The cam-lock fastening system includes a metal cam-lock and a metal pin. The cam-lock includes an engagement port to co-operate with a key to rotate the cam-lock about a rotation axis and a radial slot having an eccentric inner diameter in a plane perpendicular to the rotation axis. The pin includes a threaded aft end for attachment to a structure and a forward conical tapered end that defines an aft facing pin-to-cam-lock contact surface. The pin is threaded into a first structure and fixed. The cam-lock is received in a cylindrical opening in a second structure along the rotation axis.
To fasten the structures, the pin is roughly aligned with and inserted into an opening in the second structure along a longitudinal axis perpendicular to the rotation axis that opens into the cylindrical cam-lock opening. In an uncammed, unlocked position, the pin is inserted through the radial slot into the cam-lock. The key is inserted into the engagement port and turned to rotate the cam. The radial slot engages the pin's pin-to-cam-lock contact surface and draws the pin forward along the longitudinal axis into a fully-cammed and locked position.
The cam-lock and pin are designed such that drawing the pin forward produces an intended preload (force) in the longitudinal axis on the surfaces between the structures to clamp them together. The design assumes a certain nominal position of the pin inside the cam-lock in the unlocked position. If the pin extends too far into the cam-lock, the draw on the pin and preloading of the surface will be reduced. Conversely if the pin does not extend far enough into the cam, the draw on the pin and preloading of the surface will be increased.
The cam-lock and pin as received into their respective structures will exhibit a certain tolerance stackup along the longitudinal axis. This stackup may be more or less than the nominal position. As long as the tolerance stackup lies within the tolerance (+/−x inches) to which it was designed, the pin can be received in the cam-lock. Rotation of the cam-lock into the fully cammed and locked position fastens the structures and provides the desired preloading.
A common use for cam-lock fasteners is for the assembly of “Do It Yourself” furniture. A pin is threaded into a pre-drilled hole in a first structure and fixed. The cam-lock is placed in a pre-drilled hole in a second structure. The pin is inserted into the cam-lock, which is then rotated to the fully cammed and locked position clamping the first structure to the surface of the second structure. The tolerance stackup in this type of DIY furniture can be fairly loose, perhaps +/−0.030″ for example. The mechanics of a metal cam-lock and pin cannot absorb this amount of variation, as the metal pin cannot strain that much.
Dallara Automobili developed a system that uses a modified cam-lock fastener to rapidly attach and detach nose and wing assemblies for Indy Racing League (IRL) or INDYCAR® cars during races (See Dallara Indy Car Series 2010 Spare Parts Catalogue). As shown in FIGS. 1a, 1b and 1c, four pins 10 are threaded into pre-drilled holes on an aft portion of a nose and wing assembly 12. A pair of cam-lock assemblies 14 are mounted within a forward section 16 of the car. Each cam-lock assembly 14 includes a pair of cam-locks 18 and 20 attached to opposite ends of a cam shaft 22. The top cam-lock 18 includes a key port 24 that is accessible through a port 26 in the forward section 16 of the car. A pattern of four holes 28 (to match the pattern of pins) is formed in a surface 29 of the forward section to access the cams. The nose and wing assembly 12 is aligned to the front of the car, the pins are inserted into the holes, and a key is used to cam and lock each assembly. To remove the nose and wing assembly 12, the key is used to uncam and unlock each assembly. This approach provides a fast and reliable way to detach and attach nose and wing assemblies during a race.
Although the principles are the same, the exact cam-lock fastening system used for DIY assembly of wood furniture cannot be used to fasten the nose and wing assembly to an INDYCAR® racecar. Racecar structures are metal or composites and thus will not strain like DIY wood to absorb a tolerance stackup. Although tolerances of an INDYCAR® racecar and assembly are considerably tighter than DIY furniture, they are not tight enough. The required tolerance is in the range of +/−0.001″ to fasten a pair of metal or composite structures using a metal cam-lock and pin.
To address this problem in INDYCAR®, the pin was adapted to include an “adjustment nut” 30. The adjustment nut can be used to very precisely adjust how far the pin extends from the nose and wing assembly to achieve the tight tolerance. For an INDYCAR® race, prior to the race, crew members will adjust the adjustment nut on each pin on each of several nose and wing assemblies to be a perfect fit. To do this, the crew member must put the assembly on the car, try to lock down the assembly, remove the assembly, finely adjust the nut and repeat until camming all four of the pins can be done quickly under race conditions and provide an adequate clamping load to fasten the assembly to the car. If the pin adjustment is too long, the preload will not be sufficient to fasten the assembly and the assembly will rattle and move. If the pin adjustment is too short, the crew team might not be able to fully cam and lock the fastener. This is a highly time consuming process, but is acceptable as part of prerace preparation due to the limited number of nose assembly to car combinations.
In the defense industry, missile sections are attached and detached regularly. The fastening mechanism must provide a precise preloaded structural joint that is robust to bending moments under extreme flight conditions and provide the ability to make multiple electrical connections. The fastening mechanism must also absorb the tolerance variation due to design and manufacture of the multiple missile sections and different payloads. The current approach generally requires a large number of individual radial screws to join payload sections to the missile bodies or much less precise and costly methods like marman clamps. Additionally electrical connections that cross the sections are generally hand connected and secured with service loops in the harness. These methods are time consuming in both the production and field environment and rely heavily on operator skill and experience to meet design requirements